Patent GB-968481-A (GB)
Title: Improvements in or relating to circuit arrangements for volume compression
IPC: H03G7/02
Filed: 1960-09-30
Abstract: 968,481. Automatic volume control. PHILIPS ELECTRICAL INDUSTRIES Ltd. Sept. 30, 1960 [Oct. 2, 1959], No. 33632/60. Heading H4R. A circuit for volume compression of a band of information signals comprises a compression voltage rectifier for producing from the band of information signals a D.C. control output which amplitude modulates oscillations from an oscillator having a frequency lying outside the band of information signals, an amplitude limiter to which are applied the modulated output of said oscillator and the information signals, the amplitude of the applied modulated oscillations being greater than the applied information signals so that a series of pulses is obtained from said limiter, the duration of which pulses is modulated in accordance with the information signals, and a filter for deriving volume compressed information signals from the series of pulses. Speech signals are applied via terminals 20 (Fig. 3) through filter 21 having an upper frequency limit of 3.4 kc./s. to amplifier 23. The output of amplifier 23 is applied through transformer 25 to the control grid of amplifier 44 and through transformer 24 to compression voltage rectifiers 26, 27, to filter network 29, 30 having an upper frequency limit of 100 c/s. and to a push-pull modulator 31. This modulator 31 comprises rectifiers 35, 36 connected through transformer 37 to an oscillator (not shown) of frequency 20 kc./s. Modulator output is amplified by triode 32, passed through a pass-band filter (19.8 to 20.2 kc./s.) and applied to a limiter network. This comprises rectifiers 42, 43 in the control grid circuit of amplifier 44 and controls the amplification of the signal from transformer 25 in accordance with the speech signal level to provide volume compression. The output of amplifier 44 comprises a series of pulses, the duration of which pulses is related to the original speech signals. A low-pass filter 47 derives from these pulses volume compressed speech signals. Filter 47 has an upper frequency limit of 4 kc./s. The filtered output is amplified and applied to a carrier wave transmitter (Fig. 1, not shown). A pilot signal generator 39 is coupled to the rectifiers 26, 27 and the circuit is arranged so that if the speech signal level falls below the pilot signal level no volume compression occurs. The compression of the speech signal is determined by the pilot signal and the speech signals at the receiver may be expanded by an expansion circuit controlled by the pilot signal as described in Specification 968,482. In a modification, the frequency of the pilot frequency generator 18 (Fig. 4) is chosen so that the difference frequency between the pilot frequency and the frequency of the local oscillator 13 adjoins the speech signal band. The output of amplitude limiter 15 is connected to two branches. The first branch comprises a filter 51 having an upper limit of 3.4 kc./s. and the second branch comprises a pilot filter 54 (pass band 16.1 to 16.5 kc./s.), a frequency converter 52, and a pilot filter 53 (pass band 3.6 to 3.8 kc./s.). At the output of filter 51 compressed speech signals are obtained and at the output of filter 53 a transposed pilot signal in the frequency band 3.6 to 3.8 kc./s. is obtained adjoining the signal band of 0.3 to 3.4 kc./s.